1. Field of the Invention
The present invention relates to materials for use in magnetic recording sensors and the like, and more particularly relates to a magnetoresistive material with two metallic magnetic phases.
2. Brief Description of the Prior Art
Materials which exhibit a change in resistance when exposed to a magnetic field are of use in preparing magnetic recording sensors, such as those used, for example, in computer disk drives. At the present time, state-of-the-art computer disk drives employ sensor materials which exhibit the anisotropic magnetoresistance effect (AMR). Materials which exhibit the AMR have a magnetoresistance which depends on how the magnetic field is applied with respect to the direction of current flow.
In other types of materials, which exhibit the giant magnetoresistance effect (GMR), a non-magnetic metallic conductor, such as copper, is necessary to create a disordered state of electron spins in a ferromagnet. However, the nonmagnetic metallic conductor does not contribute to desired scattering of the conduction electrons, and in fact, may act as a low resistance shunt path which decreases the magnetoresistance. These prior GMR materials include a magnetic metal and a non-magnetic metal.
Macroscopic ferrimagnets are a new class of phase separated magnetic materials which have been recently discovered, and are described, for example, in R. J. Gambino et al., 75 J. Appl. Phys. 1871 (1994). The macroscopic ferrimagnets include two magnetic phases with a negative magnetic exchange at the phase boundary. A prototypical example is the Coxe2x80x94EuS system which has 100 xc3x85 particles of EuS in a cobalt matrix. The EuS is exchange coupled antiferromagnetically to the cobalt at least at the Co/EuS interface. In the Coxe2x80x94EuS system, the small size of the EuS particles results in a large fraction of the EuS being in close proximity to the interface which is influenced by the strong Co/EuS exchange. It has been found that these materials display unusual magneto-optical properties, as described in R. J. Gambino and P. Fumagalli, 30 IEEE Trans. Magn. 4461 (1994), and magneto-transport properties, as described in R. J. Gambino and J. Wang, 33 Scr. Metall. Mater. 1877 (1995) and R. J. Gambino et al., 31 IEEE Trans. Magn. 3915 (1995). Magnetization and Kerr hysteresis loops have confirmed the macroscopic ferrimagnetic model for these systems. In measurements of the optical and magneto-optical properties of Coxe2x80x94EuS thin films, polar Kerr rotations of up to 2xc2x0 have been observed in Co-rich films at photon energies of 4.5 eV, as described in P. Fumagalli et al, 31 IEEE Trans. Magn. 3319 (1995). Transport measurements show that the magnetoresistance of Coxe2x80x94EuS behaves like that of the widely studied granular giant magnetoresistance effect (GMR) materials, as described in S. Zhang, 61 Appl. Phys. Lett. 1855 (1992), which include particles of a ferromagnetic metal in a conductive, nonmagnetic matrix. In contrast, Coxe2x80x94EuS includes semiconducting, ferromagnetic particles in a conductive, ferromagnetic matrix of cobalt. As a consequence, the temperature dependence of the magnetoresistance is very different in the Coxe2x80x94EuS system as compared to the ordinary granular GMR materials. With respect to the magnitude of the effect, the magnetoresistivity change (xcex4xcfx81) of the Coxe2x80x94EuS system is 8xc3x9710xe2x88x925 xcexa9-cm at room temperature in a field of 1T, which is larger than other magnetoresistive materials. Even though the magnetoresistivity change of this system is large, the magnetoresistance defined as xcex4xcfx81/xcfx81 is small, typically 2xcx9c3%, because of the high resistivity of the material caused by a large volume fraction of the semiconducting EuS phase.
While materials exhibiting the AMR effect have enhanced the performance of computer disk drives, and while the aforementioned Coxe2x80x94EuS systems are promising, it would be desirable to develop materials having a larger change in resistance as a function of applied magnetic field strength, that is, a larger magnetoresistance effect. Such materials could permit the development of more sensitive magnetic recording sensors. It would be desirable to develop such materials which would exhibit the GMR as opposed to the AMR. In materials exhibiting the GMR, the resistivity decreases with the applied magnetic field independent of the direction of the applied field with respect to the direction of current flow. In addition to this desirable isotropy, the GMR is usually stronger than the AMR.
It is an object of the present invention to provide an improved material suitable for manufacturing more sensitive magnetic recording sensors.
It is another object of the present invention to provide such a material which exhibits the GMR.
It is a further object of the present invention to provide such a material which includes two ferromagnetic phases which are exchange coupled antiferromagnetically.
It is yet another object of the present invention to provide a method of manufacturing such a material.
It is a further object of the present invention to provide a method of sensing magnetic fields using such a material.
It is still another object of the present invention to provide a digital magnetic recording system which utilizes such a material in a read head.
In accordance with one form of the material of the present invention, a magnetoresistive material exhibits the GMR and has two phases. The first phase includes a matrix of an electrically conductive ferromagnetic transition metal or an alloy thereof. The second phase is a precipitate phase of an electrically conductive rare earth pnictide which exhibits ferromagnetic behavior when precipitated out of the matrix. The second phase is antiferromagnetically exchange coupled to the first phase. In a preferred form of the first embodiment, the matrix comprises cobalt and the precipitate phase comprises terbium nitride.
Thus, the present invention provides a new macroscopic ferrimagnet, in the system Coxe2x80x94TbN, which exhibits the GMR. The Coxe2x80x94TbN system demonstrates typical macroscopic ferrimagnet properties: a magnetic compensation point and negative GMR. The Coxe2x80x94TbN system with 32% TbN by volume composition shows 0.72% GMR under an applied field of 8 kOe at room temperature and 9% GMR at 250xc2x0 K under an applied field of 40 kOe. In the Coxe2x80x94TbN system, the temperature dependence of the GMR is quite different from that of ordinary GMR materials, where the negative magnetoresistance decreases with increasing temperature. The GMR in the Coxe2x80x94TbN system increases with increasing temperature, which is due to the increase of ferromagnetic alignment of the Co and TbN with an applied field caused by the decrease of exchange coupling by temperature.
In an alternative form of material according to the present invention, the second precipitate phase comprises an electrically conductive Heusler alloy such as Co2MnSn or Co2TiSn.
The present invention also provides a method of manufacturing a magnetoresistive material of the types described above. The method includes the steps of providing a target (for example, a sheet metal or thin film target) of an electrically conductive ferromagnetic transition metal or an alloy thereof; locating a plurality of pellets of an electrically conductive rare earth element (or constituents of a Heusler alloy) on a surface of the target; sputtering the target and the pellets with ions in a suitable plasma, such as an argon plasma, to cause the film and the pellets to form an amorphous alloy of the electrically conductive ferromagnetic transition metal or alloy thereof, and the electrically conductive rare earth element (or constituents of a Heusler alloy); and subsequently annealing the amorphous alloy to cause formation of the precipitate phase within the matrix of the ferromagnetic transition metal or alloy thereof. Techniques other than sputtering can also be employed.
The present invention further provides a method of detecting magnetic field strength of a magnetic field associated with a magnetization pattern recorded in a medium. The method includes the steps of providing a sensing head which includes a portion of a magnetoresistive material of the type described above; exposing the sensing head to the magnetic field of the magnetization pattern in the magnetic recording medium; sensing the electrical resistivity of the portion of magnetoresistive material exposed to the magnetic field of the magnetization pattern in the medium; and converting the electrical resistivity of the portion into a signal which is indicative of the magnetic field strength of the magnetization pattern in the medium. It will be appreciated that any of the materials of the present invention described above can be used for the portion of magnetoresistive material in the read head.
The present invention yet further provides a magnetic recording system which is adapted for use with a magnetic recording medium having a characteristic coercive force and which has a plurality of stored data in it. The data is stored in the form of a magnetization pattern in the medium. The magnetic recording system includes a write head (which is optional) and a read head. The read head includes a portion of the magnetoresistive material of the present invention which is located in proximity to the magnetic recording medium and also includes a resistivity sensor which detects the resistivity of the portion of material according to the present invention corresponding to magnetic field strength levels of the magnetization pattern in the medium. The system also includes a controller which controls the read head (and optional write head) and which converts the detected resistivity of the portion of material according to the present invention into a signal which is indicative of the stored data in the recording medium. Again, the portion of material can be any of the magnetoresistive materials according to the present invention.
These and other features and advantages of the present invention will be pointed out in the following specification, taken in connection with the accompanying drawings, and the scope of the invention will be set forth in the appended claims.